In recent years, there has been increasing demand for nonaqueous electrolyte secondary batteries as high-energy-density batteries capable of storing electrical energy. Among nonaqueous electrolyte secondary batteries, lithium-ion secondary batteries have the advantage of being light in weight and having high electromotive forces. The price of lithium resources is rising in association with the expansion of the market for lithium-ion secondary batteries. Thus, sodium ion secondary batteries containing a more inexpensive, stable sodium compound as a positive electrode active material are receiving attention.
In lithium ion secondary batteries and sodium ion secondary batteries, positive electrodes contain positive electrode active materials, conductive assistants, binders, and so forth. As positive electrode active materials, lithium compounds, such as lithium-containing transition metal oxides, are used in lithium ion secondary batteries, and sodium compounds, such as sodium-containing transition metal oxides, are used in sodium ion secondary batteries. As binders, vinylidene fluoride-based polymers, such as polyvinylidene fluoride (PVDF), are typically used. Lithium compounds and sodium compounds used as positive electrode active materials are basic, so the use of vinylidene fluoride-based polymers as binders causes gelation, in some cases.
PTLs 1 and 2 each state that an organic acid or inorganic acid is added to a positive electrode mixture in order to inhibit gelation when a vinylidene fluoride-based polymer is used in a positive electrode of a lithium ion secondary battery. In PTL 1, maleic acid, citraconic acid, or malonic acid is used as the organic acid. In PTL 2, acetic acid is used as the organic acid, and phosphoric acid, sulfuric acid, or the like is used as the organic acid.
PTL 3 states that in an electrode for a sodium ion secondary battery containing a sodium-containing transition metal compound serving as an electrode active material, a polymer that does not have a structural unit derived from vinylidene halide is used as a binder.